Reception processing for radio receivers, in which a high-frequency signal is directly sampled in a discrete-time manner, is known. Such processing is used for reducing the size and power consumption of the radio receiver and also for achieving integration of an analog signal processing section and a digital signal processing section (see Patent Literature 1 and Non-Patent Literature 1, for example).
FIG. 1 shows an overall configuration of the direct sampling circuit disclosed in Patent Literature 1. FIG. 2 is a timing chart showing control signals inputted to the circuit shown in FIG. 1. The direct sampling circuit shown in FIG. 1 converts a received analog radio frequency (RF) signal into a discrete-time analog signal by frequency conversion. This frequency conversion is performed by a multi-tap direct sampling mixer. More specifically, a filter characteristic which is a product of filter characteristics of a finite impulse response (FIR) filter and an infinite impulse response (IIR) filter is achieved by charge transfer among a plurality of capacitors provided in the circuit shown in FIG. 1. The characteristic in the vicinity of a passband is determined by a second-order IIR filter characteristic. FIG. 3(a) shows an example of a broadband frequency characteristic, and FIG. 3(b) shows an example of a narrowband frequency characteristic in the vicinity of a passband.
There is another known configuration of the direct sampling circuit that is based on the configuration described above but uses a transfer function having complex poles (see Non-Patent Literature 2), FIG. 4 shows an overall configuration of the direct sampling circuit disclosed in Non-Patent Literature 2. FIG. 5 is a timing chart showing control signals inputted to the circuit shown in FIG. 4. FIG. 6 shows an example of a frequency characteristic provided by the circuit shown in FIG. 4 (i.e., local (LO) frequency fLO=2.4 GHz). It is known that a direct sampling circuit that uses a transfer function having complex poles produces a ripple in a passband.